1. Field of the Invention
This invention relates to heavy duty pneumatic radial tires for use in heavy duty vehicles such as a truck, bus, industrial vehicle, construction vehicle and so on. More particularly it relates to a heavy duty pneumatic radial tire having an improved bead portion durability without increasing the weight of bead portion.
2. Description of Related Art
Heavy duty pneumatic radial tires used in the above heavy duty vehicles are apt to cause problems in the bead portion, mainly separation failure because a force applied to the bead portion is large during running under loading. To this end, there have been proposed various techniques for improving bead portion durability, mainly resistance to separation failure. Among them, it is well-known that the rigidity of the bead portion is further improved by an additional arrangement of a reinforcing member such as a nylon chafer, wire chafer or the like. Alternatively the rigidity distribution is corrected by the rationalization of carcass line.
These improving means are effective to control not only so-called falling deformation of the bead portion in the tire at a state of inflating under a given air pressure and at a state of loading under a given load but also the shearing deformation in the circumferential direction of the bead portion at positions corresponding to a stepping-in region and kicking-out region of the tread portion, so that they are widely adopted as a technique for contributing to the improvement of bead portion durability.
However, the additional arrangement of the above reinforcing member naturally brings about not only the increase of weight but also rise of cost due to the increase of material cost and number of production steps, so that it is unsuitable in the existing severe price competition for the tire industry Further, the other means not causing a weight increase such as the correction of the carcass line and the like certainly develop a given effect under relatively mild use conditions, but can not be adapted at the existing state of more severe use conditions and hence they are confirmed to insufficiently develop the given bead portion durability.
In a pneumatic radial tire for large-size construction vehicle among the heavy duty tires, it is usual to adopt so-called high turnup structure such that the height of the carcass turnup portion is particularly set to a level largely higher than that of the other kind of tire and sometimes set to a level at about a position corresponding to a maximum width of the tire in order to improve the cut resistance of the sidewall portion. Therefore, any conventional means attempting to improve the separation failure in the turnup portion are difficult to serve as an effectively improving countermeasure. Because a largest strain acts on a position near to a flange height of a rim during the running of the tire under the loading the large-size tire is considerably large in the bending ratio under the loading as compared with that of the other kind of tire.
In the case of the large-size tire, separation failure (problem of bead portion) is caused along a portion of the turnup ply far away from the turnup end existing in a region contacted with the rim flange due to the aforementioned large bending ratio during the running under the loading. That is, even if all of the conventional techniques inclusive of the above improving means are applied to the problems particular to the large-size tire, they are not the conclusive factor for the development of the bead portion durability satisfying commercial demand.